The present invention relates to apparatus for making webs or sheets from dilute suspensions of natural, mineral and/or synthetic fibers in liquids, especially to apparatus which can be used in papermaking or analogous machines of the type wherein a fiber slurry is fed onto or against a foraminous screen or wire to leave a thin layer or mat of fibers which can be processed into paper sheets or the like. More particularly, the invention relates to improvements in apparatus of the type wherein the width of the mat can be varied by resorting to devices known as deckles or deckle straps.
U.S. Pat. No. 3,791,919 to Schuller et. al. discloses an apparatus wherein the outlet of the head box is located above the upstream portion of the upper reach of the Fourdrinier wire and the effective width of the outlet can be regulated by two deckle straps which overlie the upper reach of the wire below the outlet and are movable transversely of the direction of lengthwise movement of the upper reach. The arrangement is such that the deckle straps are movable from the marginal portions of the upper reach of the wire toward the central portion of the upper reach to thereby reduce the width of that portion of the wire which allows the liquid (normally water) to pass therethrough. A drawback of such apparatus is that the width of the mat cannot be changed at will, i.e., that the deckle straps cannot overlie relatively large portions of the upper reach of the wire. It has been found that, if the width of the non-overlapped portion of the upper reach of the wire is reduced well below the maximum distance between the marginal portions of the wire, the density of suspension changes at an unpredictable rate. Also, excessive narrowing of the space between the deckle straps entails changes in the cross-sectional area of the mat which deposits on the wire and adversely influences the edges of the mat, i.e., the edges must be subjected to a secondary treatment which contributes to the cost of the ultimate product.
U.S. Pat. No. 1,734,929 to Vedder discloses a papermaking machine which comprises two deckle straps. The deckle straps overlie the upper reach of the Fourdrinier wire and are shiftable laterally of the wire. The width of the outlet for suspension is variable to conform to the desired width of the mat. The deckle straps of Vedder extend well beyond the outlet so that they cannot influence the width of the mat; they merely serve to prevent lateral spreading of fibers which form the mat. The behavior of fibers which are supplied onto the wire of Vedder is basically different from the behavior of fibers in a machine wherein the mat is formed within the confines of the head box, i.e., below the outlet of the device which contains a supply of suspension. The present invention relates to apparatus of the type wherein the mat or mats are formed in the head box.
It was further proposed to improve the configuration of the edges of a mat on a Fourdrinier wire by resorting to jets of a liquid which serves to trim the edges. Such mode of trimming the edges is possible only when the mat is relatively thin and consists of short fibers.